This invention relates to an optical scanning device for scanning an optical record carrier, the device comprising a radiation source for generating a radiation beam of a predetermined wavelength, which beam is directed along an optical axis of the system, and an optical diaphragm for reducing the intensity of a portion of the beam. The invention also relates to an optical diaphragm for use in scanning an optical record carrier using a radiation beam of a predetermined wavelength. In particular, but not exclusively, the invention relates to an optical scanning device for scanning an optical record carrier, such as an optical disc, in which the numerical aperture (NA) may be altered in use.
Different physical formats of optical disc are designed to be read out, and in the case of recordable or rewritable optical discs, written to, using different wavelengths and NAs. For example, standard compact discs (CDs), are designed to be read with a wavelength of 780 nm and an NA of 0.45. Standard digital video discs (DVDs), are designed to be read with a wavelength of 650 nm and an NA of 0.6. Standard rewritable DVDs (DVD+RWs) are designed to be written and read with a wavelength of 660 nm and an NA of 0.65. Standard recordable DV discs (DVRs) are designed to be written and read with a wavelength of 400 nm and an NA of 0.85.
It would be desirable to provide an optical scanning device having an optical system which is compatible with more than one of the standard physical formats. For example, a combined CD and DVD-compatible device in which the NA of the optical system is altered from 0.6 to 0.45 when the wavelength of the radiation is altered from 650 nm to 780 nm. In order to achieve this, an outer annular ring of the objective lens of the optical system may be designed to cut out radiation of 780 nm wavelength. If this is done, this outer part of the objective lens can be optimised for DVD scanning, without affecting the performance of the device when scanning CDs.
It is known to use a dichroic annular filter which cuts out 780 nm radiation but is transmissive to 650 nm radiation in a region of the radiation beam which falls on the outer part of an objective lens. However, this is a relatively expensive component. It would be desirable to provide an alternative way of altering the numerical aperture when radiating different wavelengths in an optical scanning device.
It would also be desirable to provide a way of selectively switching between different NAs of an optical system, without necessarily altering the wavelength of radiation. It is for example possible to adopt a scanning device to read different formats of optical disc by only altering the NA of the optical system, without altering the wavelength so that a single wavelength laser system may be used.
International Patent Application WO 97/21215 describes an optical scanning device for optical discs, which is capable of reading and/or writing both CDs and DVDs. A diaphragm is formed by a polarising filter which is associated with the objective lens. To switch between the different formats, a half-wave plate, along with a divergent lens to correct for spherical aberrations created by different information layer depths in the different formats of discs, is mechanically inserted or removed from the optical path of the system. The half-wave plate alters the linear polarisation of the radiation by 90xc2x0 and therefore switches between the different numerical apertures provided by the polarising filter. However, the mechanical insertion and removal of the half-wave plate requires mechanical actuators which complicate the design of the optical system and tend to increase the expense of manufacture of such a system.
JP-A-10143900 describes an optical disc pick-up device having means for altering a numerical aperture of the radiation beam radiated onto the optical disc, which is similar to that described in WO 97/21215.
JP-A-1086334 describes an optical scanning advice in which the numerical aperture of the beam radiated onto an optical disc may be altered by means of selectively diffractive optical elements based on a control signal. It would be desirable to provide an alternative to such an arrangement.
EP-A-0932145 describes an optical scanning device in which the numerical aperture of the beam radiated onto an optical disc may be altered selectively, in dependence on the wavelength of the radiation being used. A polarisation plate having an annular polarised region acts as a polarisation-selective optical diaphragm. A radiation source capable of producing two different wavelengths of radiation beam, at different polarisations, is used. It would be desirable to provide an alternative arrangement.
In accordance with one aspect of the present invention there is provided an optical scanning device for scanning an optical record carrier, said device comprising a radiation source for generating a radiation beam of a predetermined wavelength, which beam is directed along an optical axis of the device, and an optical diaphragm for reducing the intensity of a portion of the beam, characterised in that said optical diaphragm includes a cholesteric liquid crystal material having a helical pitch selected to provide internal reflection of said portion of the beam.
In accordance with a further aspect of the invention there is provided an optical diaphragm for use in scanning an optical record carrier using a radiation beam of a predetermined wavelength, said optical diaphragm being for reducing the intensity of a portion of the beam, characterised in that said optical diaphragm includes a cholesteric liquid crystal material having a helical pitch selected to provide internal reflection of said portion of the beam.
Cholesteric liquid crystals, in common with other types of liquid crystal, are formed of long molecules which on average lie in one direction. This direction is referred to as the director, and is indicated by means of a vector n. Although the molecules do not exhibit symmetry between their two ends, on average as many molecules are aligned in the direction of n as in the opposite direction. Therefore, n and xe2x88x92n are equivalent. Nematic liquid crystals without external influence naturally adopt an arrangement in which the director is uniform throughout. In contrast, cholesteric liquid crystals, without any external influence naturally adopt rangement in which the director varies helically in direction about a helical axis. In each plane perpendicular to the helical axis, the director is uniform and parallel to the plane. The angle between the director and a reference director perpendicular to the vertical axis varies linearly along the helical axis. The distance over which the director turns through 360xc2x0 is referred to as the helical pitch p.
Similar to other liquid crystals, cholesteric liquid crystals are birefringent. Radiation with a linear polarisation parallel to the director experiences a refractive index ne, whereas radiation with a linear polarisation perpendicular to the director experiences a different refractive index no. A cholesteric liquid crystal layer can exhibit various optical properties depending on the refractive indices ne and no and the vertical pitch p of the cholesteric material, the wavelength xcex and angle of incidence xcex8 with respect to the helical axis of the material of the incident radiation and the thickness of the layer. One such property is polarisation-selective (substantially total)-internal reflection. When xcex8=0, a reflection band occurs between xcexmin=nop and xcexmax=nep. The polarisation-selective nature of the reflection lies in the fact that only one circular polarisation is reflected, that having the same handedness as the cholesteric helix. The layer is (substantially fully) transmissive to radiation of the opposite circular polarisation. When xcex8xe2x89xa00, the reflection band occurs between
xcexmin=nop{square root over (1xe2x88x92sin xcex82/{overscore (n)})}2 and xcexmax=nep{square root over (1xe2x88x92sin xcex82/{overscore (n)})}2, where {overscore (n)}=(no+ne)/2.